I have a mid 1930's BTH singles mag that worked after a fashion till i cleaned and adjusted the points and earth brush. The gap was larger than it should be ,possibly 20 thou. Now its 12 thou the spark has gone. I think the internal timing must be out and the large gap was used to advance the mag internal timing . This also advanced the ignition timing making the bike hard to start and kick back. Any mag whiskids on our forum?.I would like to investigate a bit before sending it off. These mags do not keep their magnetism if dismantled,unlike a later Lucas.
Hi Robert You are correct…
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Hi Andy, the points have b…
Hi Andy, the points have been in and out a few times ,I'm going to do some continuity and resistance tests and then try it with the points wider. If that does not work I'll remove it from the machine for some bench testing.I think it was sparking on full advance but no chance to start like that ,violent kickbacks.
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Hi Robert
You are correct in thinking that closing the points gap will retard the internal timing, but by such a small amount that it is unlikely to stop it sparking. There is also very little scope to adjust the internal timing on the BTH so I'd be surprised if it was wrong in the first place. You want the points to open just after the magnet lets go, being over advanced will stop it sparking a lot sooner than being too far retarded (just think about when you pull the lever to retard it for starting, you could be moving it 20+ degrees, your point adjustment may have moved it 1degree).
Have you checked the points are clean? I have seen several mags stop working after someone cleans the points, if you leave behind a fibre or two of whatever you used to clean them you can stop the points making contact when closed. Easily done!
Good luck
Andy